This notebook was prepared by Donne Martin. Source and license info is on GitHub.

# Solution Notebook¶

## Constraints¶

• Can we assume the string is ASCII?
• Yes
• Note: Unicode strings could require special handling depending on your language
• Is this case sensitive?
• Yes
• Can we use additional data structures?
• Yes
• Can we assume this fits in memory?
• Yes

## Test Cases¶

• Any strings that differ in size -> False
• None, 'foo' -> False (any None results in False)
• ' ', 'foo' -> False
• ' ', ' ' -> True
• 'foobarbaz', 'barbazfoo' -> True

## Algorithm¶

Examine the following test case:

• s1 = 'barbazfoo'
• s2 = 'foobarbaz'

We see that if we can use the given is_substring method if we take compare s2 with s1 + s1:

• s2 = 'foobarbaz'
• s3 = 'barbazfoobarbazfoo'

Complexity:

• Time: O(n)
• Space: O(n)

## Code¶

In [1]:
class Rotation(object):

def is_substring(self, s1, s2):
return s1 in s2

def is_rotation(self, s1, s2):
if s1 is None or s2 is None:
return False
if len(s1) != len(s2):
return False
return self.is_substring(s1, s2 + s2)

## Unit Test¶

In [2]:
%%writefile test_rotation.py
import unittest

class TestRotation(unittest.TestCase):

def test_rotation(self):
rotation = Rotation()
self.assertEqual(rotation.is_rotation('o', 'oo'), False)
self.assertEqual(rotation.is_rotation(None, 'foo'), False)
self.assertEqual(rotation.is_rotation('', 'foo'), False)
self.assertEqual(rotation.is_rotation('', ''), True)
self.assertEqual(rotation.is_rotation('foobarbaz', 'barbazfoo'), True)
print('Success: test_rotation')

def main():
test = TestRotation()
test.test_rotation()

if __name__ == '__main__':
main()
Overwriting test_rotation.py
In [3]:
%run -i test_rotation.py
Success: test_rotation